We have defined three separate regulatory mechanisms with specialized functions that act at different points in the development of cytotoxic T-lymphocytes (CTL). Alloantigenic stimulation induces two types of T-suppressort cells (CTLs). A third regulatory component in CTL development is antigen-antibody complexes (AAC). When cocultured with normal lymphocytes and antigen, both types of CTLs inhibit the development of a primary CTL response by normal CTL precursors (CTLp), as measured in a standard 51Cr-release cytotoxicity assay. Limiting dilution analysis after coculture reveals that one of the CTLs inhibits clonal expansion of CTLp, whereas the other allows proliferation but suppresses the differentiation of CTLp into CTL effector cells (CTLe). These two types of CTLs are induced selectively by different MHC determinants - the former by K/D + I alloantigens, and the latter by K/D inthe absenceof I. AAC modulates the secondary CTL response. This study will delineate just which MHC determinants induce the two CTLs subsets, other features which distinguish them and the site of action of AAC and its possible interaction with CTLs in the secondary CTL response. The information to be gained will define an experimental model in which to analyze the specificity, properties, functions, and interactions of immunoregulatory elements that modulate cell-mediated immunity. Results from this research will suggest effective means for induction of allograft tolerance. CTLs and AAC are also present in cancer patients. Findings from this study may lead to therapies for counteracting the negative influences of AAC and CTLs on anti-tumor immunity.